This invention relates to secondary batteries and, in particular, to zinc negative electrodes for use in such batteries.
A significant class of secondary batteries such as, for example, silver-zinc, zinc-oxygen and, in particular, nickel-zinc look to the incorporation of zinc active material in the negative battery electrode. However, use of a zinc negative electrode has been found to result in limited battery life, owing to the inability of the zinc electrode to effectively operate for a large number of battery charge-discharge cycles. A primary cause of this short cycle life is the solubility of the zinc active material in the battery electrolyte. Such solubility results in a redistribution of the zinc active material on the electrode current collector (so-called "shape change") and growth of zinc dendrites in the cell. The former leads to passivity of the zinc electrode and the latter to shorting of the zinc electrode and its associated positive electrode, the battery in either case being rendered inoperative. These effects are further aggravated in batteries, such as, for example, nickel zinc batteries, where overcharging is required to maintain the electrical capacity of the positive electrode during cycling.
A further factor contributing to limited zinc electrode cycle life is the swelling of the battery positive electrode. Such swelling is particularly pronounced in nickel-zinc cells and results in considerable pressure being exerted on the zinc electrode. The electrode is thereby reduced in thickness and loses electrolyte (is dried), whereby passivity and failure of the electrode are accelerated.
The prior art has attempted to counter the aforesaid undesirable characteristics of zinc electrodes in a variety of ways. In particular, electrode constructions adapted to alleviate the undesirable phenomena attributable to zinc solubility, have been disclosed in a number of U.S. patents. In U.S. Pat. No. 3,873,367, electrode constructions are disclosed wherein constituents such as calcium or magnesium hydroxide are incorporated in the zinc active material for reacting with same to form zincates which are substantially insoluble in the cell electrolyte.
U.S. Pat. No. 4,022,953, assigned to the same assignee hereof, discloses further electrode constructions wherein cadmium is added to the zinc active material to provide inert nucleation cites for zinc to replate during charging. U.S. patent application Ser. No. 163,935, filed June 27, 1980 teaches an electrode construction wherein nucleation cites for replating of zinc are provided by zinc titanate or zinc fluoride additives to the active material.
While these electrode constructions have provided some measure of improvement with regard to zinc solubility, battery designers are searching for constructions which exhibit still more improvement. Furthermore, constructions are being sought which offer resistance to positive electrode swelling.
It is therefore an object of the present invention to provide a negative zinc electrode adapted to overcome the shortcomings of prior art zinc electrodes.
It is a further object of the present invention to provide zinc electrode which is resistive to shape change and dendritic growth.
It is yet a further object of the present invention to provide a zinc electrode which resists the effects (thickness reduction and drying) of positive electrode swelling.
It is still a further object of the present invention to provide a zinc electrode for a nickel-zinc battery.
It is a further object of the present invention to provide a zinc electrode for use in alkaline batteries which has extended life during cycling due to improved physical structure and insensitivity to continuous overcharge.